When Princess Adora became She-Ra, her sword's magic transformed her horse Spirit into a winged unicorn named Swift Wind. Swift Wind is able to talk to Sher-Ra and can fly at incredible speeds. He almost as strong as she is, and can fly pulling extremely heavy loads. He can change from horse to unicorn anytime She-Ra transforms.

Origin #1 (Filmation /MOTUC Bios):

A magical steed from Etheria, Spirit served Adora during her time as Force Captain of the Horde Army. Loyal to Adora, not the Horde, Spirit stayed with her after Adora learned of her true heritage and joined the Great Rebellion. Enhanced by Adora's newly acquired Sword of Protection, Spirit was transformed into She-Ra's flying mount, Swiftwind, heroically carrying the Princess of Power into battle. Swiftwind remained with She-Ra, traveling with her from Etheria to Eternia to join with the renegade Masters of the Universe in the continued fight against Hordak and his armies.

Origin #2 (DC 52 Comics):

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Trivia (Clawful94): Swift Wind was voiced by Lou Scheimer in the Princess Of Power, Filmation cartoons.